The present invention relates generally to reducing artifacts in a sampled physiological signal from a subject. More specifically, the present invention relates to methods and systems for delivering stimulation in separate epochs of time from data acquisition epochs of time.
Closed loop neurological stimulation systems that acquire physiological signals from a subject and stimulate a subject can produce stimulation-induced artifact signals when the acquisition of the physiological signal from the subject and stimulation of the subject occur substantially at the same time.
For example, electroencephalographic (EEG) signals acquired from subjects are small in amplitude and relatively low in frequency. Scalp EEG signals, for example, have amplitudes on the order of about 10 uV to about 100 uV and have frequency content in the range of about 0.1 to about 70 Hz. Electrocorticography (ECoG) is the practice of recording signals directly from the surface of the cerebral cortex. ECoG signals are larger, in the range of about 10 uV to about 1000 uV, and depending on the size and geometry of the measuring electrode, which may vary from a disk contact several mm in diameter to a needle-like micro-electrode with dimensions on the orders of microns, these signals may contain useful information at frequencies from 0.01 Hz up to up to several hundred Hz. In practice, the term EEG or intracranial EEG is often used in place of the term ECoG; for simplicity we will use the term “EEG” throughout the remainder of this document to encompass both intracranial EEG and scalp EEG.
Electrical stimulation signals used to stimulate nerve cells are typically similar in frequency to EEG signals but can be much larger in amplitude. Typical stimulation frequencies, or pulse repetition frequencies, range from about 1 Hz to about 1000 Hz. Stimulation pulse widths are typically on the order of about 50 us to about 500 us. Pulse shapes are typically rectangular rather than smoothed or sinusoidal, and therefore generate significant amount of energy at harmonics of the pulse repetition frequency. Typical stimulation waveform amplitudes may be in the range of about 1 V to about 10 V, and when coupled to stimulating electrodes with typical electrode impedances of about 500 Ohms to about 2000 Ohms, this leads to stimulation current amplitudes of between about 0.5 mA and about 20 mA. In order to avoid damage to neural tissue or to the stimulating electrodes, the charge per stimulation pulse should be limited to below about 100 uC to about 200 uC per square cm of electrode area.
A stimulation signal may cause artifact when sensed at substantially the same time as a physiological signal because the electrical stimulation signal can be substantially larger and in substantially the same frequency band as the physiological signal (e.g., an EEG signal).
Stimulation-induced artifact can be reduced by filtering out the artifact signal. Some existing technologies may also synchronize the timing of the filtering process with the timing of the stimulation signal in order to reduce stimulation-induced artifact. These systems, however, do not control the time at which the signal acquisition and stimulation occur, but rather rely on filtering out the artifact signal after it has already been acquired. In addition, some systems simply disable the detection device while stimulation occurs.
There remains a need for a neurological stimulation system which acquires physiological signals from a subject and stimulates the subject to reduce the amount of stimulation-induced artifact sensed by the system.